It is well-known to sample an electrical input signal by use of a sampling bridge. The conventional sampling bridge has a Schottky diode in each arm of the bridge. The input signal is applied to one corner of the bridge, and strobe pulses of opposite polarity are applied to the two corners adjacent the input corner. A quantity of charge proportional to the amplitude of the input signal at the time of application of the strobe pulses is stored on a capacitor that is connected to the remaining corner of the bridge.
Another type of sampler that is commonly used is the travelling wave sampler, in which the input signal is applied to each of two strings of series-connected Schottky diodes, the diodes of the two strings being connected with opposite polarity. A short delay line is interposed between the second and third diode of each string. In the steady state, the diodes are reverse biased. Strobe pulses overcome the reverse bias and allow signal charge to be injected into the delay lines. At the end of the strobe pulses, the diodes become reverse biased once more, and the signal charge is trapped on the delay lines. The trapped signal charge has a common mode component that is proportional to signal amplitude.
Both of these conventional samplers suffer from the disadvantage that Schottky diodes are susceptible to destruction due to electrostatic discharge. Moreover, blowby (transfer of signal charge through a diode when in the reverse biased state) influences the charge stored on the delay lines in the travelling wave sampler, resulting in distortion of the output signal generated from the quantity of charge stored on the delay lines.
An electro-optic sampler does not employ Schottky diodes in the sampling bridge, and therefore is not subject to the problems associated with those diodes. An electro-optic sampler comprises a pulsed source of polarized light, a photodetector device that generates an electrical signal representative of the intensity with which light polarized in a predetermined manner is incident on the photodetector device, and a body of electro-optic material defining an optical waveguide for transmitting light from the source of polarized light to the photodetector device. The body of electro-optic material has first and second electrodes for establishing an electrical field within the body of electro-optic material. As the light source is pulsed, the photodetector device provides an output signal representative of the field applied to the body of electro-optic material. Therefore, if one of the electrodes is grounded and the other electrode receives a signal to be sampled, the electrical signal provided by the photodetector device represents the instantaneous value of the input signal at each strobe.
Although the electro-optic sampler avoids the disadvantages of the conventional diode sampler that are associated with use of Schottky diodes, it is subject to other disadvantages. In particular, conventional laser diodes exhibit phase noise (variations in the wavelength of the light generated by the laser diode), and the static birefringence of the electro-optic material results in undesirable amplitude noise in the output signal.
The dynamic range of an input signal that can be sampled accurately is greater if error sampling is employed than if absolute value sampling is employed. In a conventional travelling wave error sampler, the signal charge captured on the delay lines during a strobe pulse is applied to an amplifier. The output of the amplifier is fed to a gated memory, which includes a memory capacitor and a memory gate switch. The switch is rendered conductive and non-conductive at the strobe rate. The voltage that is developed on the capacitor while the memory gate switch is conductive due to a first strobe pulse is used to bias the diodes of the sampler and the additional signal charge captured on the delay lines during the next strobe pulse is then dependent on the difference between the signal amplitude at the time of that strobe pulse and the signal amplitude at the time of the first strobe pulse. Further information concerning travelling wave samplers and memory bridges can be found in co-pending applications Ser. Nos. 06/845,900 and 06/851,859, now U.S. Pat. Nos. 4,647,795 and 4,659,946.
Hitherto, error sampling has not been used in conjunction with an electro-optic sampler.
A conventional laser diode has two opposite facets from which light is emitted substantially simultaneously when the diode is pulsed. The two beams are of substantially equal intensity. Conventionally, the beam emitted from one facet of the laser diode is used to act on an external agent or medium and the beam emitted from the other facet is used to monitor the power output of the laser diode.